This invention relates to the recovery of olefinic compounds from streams comprising olefinic compounds, especially the recovery and recycle to polymerization zones of such olefinic compounds. This invention also relates to a process for the removal of hydrogen from streams comprising olefinic compounds.
As is well known, the polymerization of olefinic compounds is a commercially important process in view of the multitude of uses of polymers and copolymers. Two of the most commercially important polymers are polyethylene and polypropylene; however, a myriad of other polymers and copolymers have been found commercially important. For example, ethylene is not only often polymerized with itself to form polyethylene, but also with other olefinic materials such as propylene, alpha olefins, acrylates, vinyl acetates, and the like. Therefore, for the purposes of this disclosure, the term polymerization shall include copolymerization.
Commonly the olefinic compounds or mixture of olefinic compounds to be polymerized is passed into a reaction zone where polymerization takes place. Generally a catalyst is used to promote such polymerization. Commonly a quench solvent such as a saturated hydrocarbon, for example isobutane or isopentane, is added to cool the contents of the reaction zone and thereby control the reaction. As is well known the polymerization of olefinic compounds is generally a highly exothermic reaction and careful control of temperature must be maintained. In numerous processes hydrogen is added to the reaction zone to help control the molecular weight of the polymer or polyolefin.
Feed olefinic compound to polymerization processes commonly contains impurities such as saturated materials having approximately the same molecular weight as the olefin. For example, ethylene commonly contains a small amount of ethane and in some cases smaller amounts of methane and propane. Propylene commonly contains small amounts of propane and to a lesser degree ethane and butane. Additional saturated materials can be formed in the reaction zone by reaction of the hydrogen which was added to control molecular weight of the polymer, with the olefin feed to the reactor. The olefinic compound is hydrogenated and becomes saturated, thereby forming the corresponding saturated compound of the olefin. Therefore, it can be seen that there are numerous sources of saturated material in the polymerization process. These saturated materials generally do not take part in the polymerization process but merely pass through the process unreacted. While in many cases these saturated materials are not detrimental to the polymerization reaction itself, reactor capacity can unnecessarily be used in passing through these inerts. These saturated materials are removed with final product and unreacted olefin from the reaction zone. In most processes, the unreacted olefinic compound and saturated contaminant is recycled to the reactor. As is taught in Steigelmann et al, U.S. Ser. No. 948,332 filed concurrently with this application, in order to prevent concentration of the contaminant in the reactor, a portion of this recycle stream can be sent to a membrane separation unit to separate olefinic compound for use, such as recycle to the polymerization zone. Membrane separation units often use metal containing ions to facilitate transport of olefinic compounds across a membrane. Hydrogen is commonly present in certain streams such as streams from polymerization units using hydrogen for molecular weight control. Membrane separation units can be adversely affected by hydrogen, as by changing the pH of aqueous solutions concentrated therein or reducing certain metal containing ions. One such ion, silver, can be reduced to silver metal by hydrogen thereby plugging the membrane and reducing the selectivity of the separation process. Therefore it is desirable to remove hydrogen from the stream passed to the membrane separator to prevent such detrimental effects.
It is an object of this invention to recover olefinic compound from streams comprising olefinic compound, especially those comprising olefinic compound and hydrogen.
It is an object of this invention to recover olefinic compound from streams from polymerization zones for recycle back to such polymerization zone.
It is an object of this invention to provide a method for removing hydrogen from streams containing olefinic compounds.
It is an object of this invention to provide an improved method of separating olefinic compounds from mixtures containing them.
It is an object of this invention to remove hydrogen from streams without accumulating hydrogen for disposal.